1. Field of the Invention
The present invention relates to a video decoding device and in particular but not exclusively to a video decoding device for decoding Moving Picture Experts Group (MPEG) and associated encoded video types.
2. Discussion of the Related Art
Digital video compression is used in many different environments and has become the first choice in many different situations. Three such environments are: broadcasting and the DVB (Digital Video Broadcasting) standard; storage of video data and the DVD (Digital Versatile Disc) standards; and internet node to node communication using standards such as MPEG-4 encoding.
Traditionally most of these coders and decoders (Codecs) associated with these are based on the original MPEG (Moving Picture Experts Group—the international standards organization/international electro-technical commission video decoding and encoding methods) standards, commonly known as MPEG2. Codecs and the coding/decoding algorithms powering them have improved since the original algorithms were set as the standards. These improvements have been incorporated into a series of Codecs which while not being MPEG standards still use MPEG type segmentation and compression steps.
MPEG and MPEG type codecs treat video data which consists of a series of images—these images are known as pictures. There are two types of picture, a frame picture or a field picture. A frame picture is further subdivided into a pair of fields, each field comprising alternate rows of a scanned image. A field picture contains the rows of image data from a single field. Each picture can be treated by the codec as a reference picture or as a non reference picture.
All of these MPEG type schemes encode and decode with reference to the frame or field pictures and in particular to a specific reference or multiple reference pictures. In its most basic form this involves encoding an initial reference picture followed by a series of non-reference pictures. In practice however the distinction between reference and non-reference pictures is not as simple. Reference pictures can be both intra (I) pictures and/or P pictures. An I picture is a picture which is encoded without reference to any other picture. Both P and B pictures are encoded however using data from one or more other pictures. B pictures are not used to encode other pictures.
The I pictures are broken down into a series of macro blocks (MB). The blocks are then processed. The B and P pictures search the macro blocks from the P and I pictures to find similar blocks to those in the B and P pictures. The B and P pictures are then encoded in terms of macro blocks which differ from the macro blocks of the I and P pictures in terms of content and also position.
Decoding schemes perform the reverse of the encoding scheme, that is, recreating the original P and I pictures, and then using the I and P pictures and extra P and B picture data to reconstruct the original P and B pictures.
Current commercially usable video decoding chip solutions have been designed primarily to decode MPEG2 coded video streams. These commercially available solutions are therefore MPEG2 d optimized and are not easily capable of being modified to decode video streams encoded using any other standard.
Known flexible decoders are available but have been based on the general processing unit (GPU) such as those used within personal computers. These flexible decoders perform the whole decoding process using instructions stored in memory external to the processor, which is selected and loaded dependent on the video standard required to be decoded. These GPU's are not d optimized to perform video decoding, consume a large amount of power and produce a large amount of heat when compared against the previous MPEG2 video processing units used. The GPU solution is also expensive compared against the MPEG2 video processing units previously used and therefore making the cost of production of a flexible video decoding system expensive.